Passive nuclear detection systems span a wide range of applications for homeland security employing both photon and neutron detection systems. The applications range from portable hand-held units to portal monitors located at ports of entry and at weigh stations along transportation corridors. Passive nuclear detection systems are used for standoff detection of special nuclear materials for search applications, imaging of special nuclear materials, radioisotope identification, and container and vehicle screening, just to name a few.
Over 7 million cargo containers enter U.S. ports each year. However, less than 2% of the actual containers are surveyed for the presence of radioactive materials when they arrive. The U.S. Department of Commerce anticipates the number of cargo containers entering the U.S. to quadruple over the next 20 years. This high volume of material movement is a significant challenge for interdicting any attempted shipment of nuclear material into the U.S because a balance between security and commerce must be established. Radiation portal monitors (RPMs) used in ports are mostly comprised of plastic scintillation detectors and some have additional neutron detectors that monitor containers by looking for counts that exceed a threshold. These portal monitors have cost that can exceed $150K. However, these systems are susceptible to variations in background radiation. Additionally, naturally occurring radioactive material (NORM) commonly encountered in commerce further exacerbates the problem. See Valentine, Overview of Nuclear Detection Needs for Homeland Security, http://www.ornl.gov/˜webworks/cppr/y2001/pres/125015.pdf, 2001, Oak Ridge National Laboratory Website.
For these systems, there are many technological solutions for detecting gamma rays. Most of these rely on scintillation detectors and semiconductors. Inorganic scintillation detectors are commonly chosen as gamma-ray transducers because of their high-Z value and density. Additionally the light output of inorganic scintillation detector is more linear than that of organic scintillation detectors. Inorganic scintillation detectors are commonly fabricated using single-crystal growth methods such as the Bridgman or Czochralski techniques. However, while promising new materials have been grown using these techniques, like Sodium Iodide, NaI, and high purity Germanium, HGe, crystal growth continues to be a time-consuming and expensive method for production of scintillation materials for radiation detectors. Additionally, most single crystals are limited in size thereby imposing constraints on the final radiation detectors. Glass and ceramic scintillation detectors offer the potential for the fabrication of relatively inexpensive and plentiful detectors, yet glass scintillation detectors have suffered from relatively low light output. Ceramic scintillation detectors have received less attention over the past few decades but interest has been growing in the development of transparent polycrystalline ceramic materials.
Advanced spectroscopic portals (ASPs) have been deployed that have the capability to distinguish NORM from special nuclear materials. However, cost and production capacity limit the widespread deployment of ASPs. Handheld and portable radiation detection systems are used as a supplement for radiation portal monitors in some cases. These handheld and portable radiation detection systems can be gross counting systems or spectroscopic systems. The limitations of existing spectroscopic systems, be it resolution or the need for cryogenic cooling, require development of new detection systems based on new or improved detection materials or alternate cooling systems such as thermoelectric coolers. For high-resolution gamma spectroscopy measurements, the goal for energy resolution at full-width half maximum is less than 0.5% at 662 keV for a room temperature scintillation detector. This goal cannot be achieved with existing room temperature detectors; for example, the resolution for state of the art CZT detectors is approximately 1.7% at 662 keV (with very low detection probability).
Active nuclear detection systems are also commonly used for radiography or for secondary inspection. These systems employ neutron or gamma rays sources or both to either provide a detailed image of the cargo container, or to specifically identify special nuclear materials in cargo or containers. Such systems have been deployed at U.S. ports of entry and are used to further characterize items that are removed from containers. Transmission radiography is commonly employed to image suspect containers or containers chosen at random. These systems typically use an x-ray generator or mono-energetic gamma ray source to provide images of high-density and low-density materials in containers. Such systems are either fixed or mobile depending on the needs at the particular location.
Transmission radiography technologies may be useful to detect high Z materials that are often used in shielding, but these measurements do not verify the presence of special nuclear materials (SNM). Additionally, these systems cost nearly $1M each. The detection and verification of SNM is an important challenge for the Government. While advances in spectroscopic measurement systems will greatly enhance the probability of detecting nonshielded or weakly shielded SNM, the detection of shielded SNM and in particular shielded high-enriched uranium (HEU) poses a significant challenge that is best addressed using active detection systems.
Active interrogation techniques utilize both neutron and gamma ray sources and includes nuclear resonance fluorescence, neutron and gamma ray multiplicity, neutron radiography, and neutron and gamma ray induced fission. These systems can be utilized to inspect cargo in shipping container at seaports and border crossings, air transport containers, or to be deployed as mobile inspection systems. To support the development of these systems, additional research and development is needed in neutron and gamma ray sources, detection models, neutron and gamma ray emission data, and neutron and gamma ray simulation codes.
For security purposes, would be very helpful to have a radiation detection system for in-transit monitoring of containers or vehicles. These systems would have to be small, low-cost, tamper proof, and provide reasonable probability of detection of materials of interest. An extremely low false alarm rate would be a necessity as an alarm from one of these on-board sensors would require the ship to be delayed from docking. Hence, such in-transit sensors would need spectroscopic capability to distinguish special nuclear material from NORM and cosmic ray induced radiation.
Therefore, there exists a need for a remote directional detector/imager and identifier of special nuclear material and other radiation sources which can detect a radiation source at speed through commerce without the need to stop the commerce. The present invention addresses this need.